Download Imaging thermal ion mass and velocity analyzer

An Introduction to Space Instrumentation,
Edited by K. Oyama and C. Z. Cheng, 203–215.
Imaging thermal ion mass and velocity analyzer
Andrew W. Yau, E. Peter King, Peter Amerl, Kaare Berg, Greg Enno,
Andrew Howarth, Ivan Wevers, and Andrew White
Department of Physics and Astronomy, University of Calgary,
2500 University Dr NW Calgary, Alberta T2N1N4, Canada
The aim of an imaging thermal ion mass and velocity analyzer is to apply imaging techniques to measure
in-situ the mass composition and detailed velocity phase space distributions of a thermal plasma population in a
planetary ionosphere or magnetosphere and use the measured distributions to derive the bulk plasma parameters
and to detect the possible presence of non-thermal distributions. A hemispherical electrostatic analyzer (HEA)
with a planar entrance aperture can sample simultaneously incident ions or electrons over an extended energy
range and the full 360◦ range of incident azimuth, and disperse them by their energy-per-charge while retaining
their incident azimuth, thus providing a means to image the 2-dimensional (2D) ion or electron energy-per-charge
and angular (azimuth) distribution. Therefore an ion mass and velocity analyzer consisting of a HEA embedded
with an ion-mass spectrometer is capable of imaging the 2-D detailed ion velocity distribution—and measuring
the 3D distribution on a spinning spacecraft if the planar entrance aperture is aligned along the spacecraft spin
axis. For 3D velocity distribution measurements on a 3-axis stabilized spacecraft, an analyzer with electrostatic
deflection capability will be required to deflect ions at arbitrary incident elevation angles into the planar entrance
aperture for sampling.
An imaging thermal ion mass and velocity analyzer is presented that combines a HEA, a time-of-flight ion
mass spectrometer, and a pair of electrostatic deflectors, and is capable of sampling low-energy ions (∼1 to
100 eV/e) of all mass species (1 to > 40 AMU/e) from all incident directions on a non-spinning platform, at
up to (10% energy resolution (E/E) and ∼5◦ angular resolution. Using the HEA to measure the energy-percharge of each detected ion and the time-of-flight gate to measure the transit time of the ion inside the analyzer,
this instrument can resolve all major ion species in the ionosphere including H+ , He+ and O+ , and adjacent
+
+
molecular ion species such as N+
2 , NO and O2 under favorable conditions. In addition, it can image the 2D and
measure the 3D velocity phase space distributions of each major ion species.
Key words: Ion mass spectrometer, plasma analyzer, hemispherical analyzer.
1.
Introduction
The terms “thermal”, “core” and “low-energy” are often used interchangeably in the literature to refer to the
lowest-energy, core component of a plasma population in
a planetary ionosphere or magnetosphere. Both the mass
composition of this thermal plasma and its detailed velocity phase space distribution often play an important role in
the structure and the dynamics of a planetary ionosphere
or magnetosphere. An imaging thermal ion mass and velocity analyzer aims to apply imaging techniques to obtain
both of these quantities in-situ, by measuring the macroscopic ion mass composition distribution and the detailed
mass-resolved velocity phase space distribution of each ion
species. The goal is to use the measured distributions to
derive the bulk plasma parameters—ion composition, density, bulk velocity and temperature—and to detect the possible presence of non-thermal distributions (deviations of
the velocity phase space distribution from thermal distributions), for studying plasma acceleration and transport processes in different regions of a planetary ionosphere or magnetosphere.
The structure and dynamics of the ionosphere on a
weakly-magnetized or non-magnetized planet such as Mars
c TERRAPUB, 2013.
Copyright and Venus depends strongly on the nature, magnitude and
topology of its intrinsic induced magnetic field, and its direct interaction with the solar wind is believed to result in
a significant escape of the planetary atmosphere and play a
key role in the latter’s evolution over geological time. In the
case of Mars, for example, thermal ion composition (Hanson et al., 1977) and cold ion measurements (Dubinin et
al., 1993) have been key to our current knowledge on its
ionospheric composition and dynamics and its solar wind
interaction.
In the case of the Earth, the thermal plasma plays an important role in the coupling between the ionosphere and
the magnetosphere: As discussed in, for example, Chappell (1988) and Yau and Andre (1997), the magnetosphere
often contains a significant component of “cold” ions originating as ion outflows from the ionosphere. In the polar
ionosphere, both ion density and mass composition are important parameters in ionospheric ion acceleration and outflow; for example, the ion mass density affects the properties of Alfven waves, which carry field-aligned currents and
plasma waves (Lysak and Lotko, 1996), and the plasma density is believed to control auroral acceleration and auroral
kilometric radiation (AKR) (Morooka and Mukai, 2003).
The ion bulk flow velocity is also an important plasma parameter due to its direct (E × B) relationship with the convection electric field in the “frozen-in” and collision-less
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Fig. 1. Schematic illustration of a cylindrical electrostatic analyzer (top),
which measures ions at one ion energy-per-charge and one incident
azimuth at a time using a channel electron multiplier detector, and
the rotational cross-section of a top-hat electrostatic analyzer (bottom),
which measures simultaneously ions at a fixed energy-per-charge over
the full 360◦ range of incident azimuth (in and out of the page) using
a bias voltage (V ) between the inner and outer analyzer surface and a
micro-channel plate detector (denoted by the solid rectangle).
regions of the magnetosphere and the ionosphere, where
large-scale convective electric fields play a critical role in
plasma circulation, redistribution and energization. Many
of the existing techniques for measuring thermal ions are
based on electrostatic or retarding potential analysis (RPA)
and focused on resolving the energy and angular distributions of the measured ions. In many instances these techniques are also applicable to electron measurements. Some
of the existing techniques also resolve the mass composition
of the measured ions. The earliest ion composition and density measurements in space date back to those using Bennett
or open ion mass spectrometers (Brinton et al., 1973) and
retarding potential analyzers (Hanson et al., 1973) on the
Earth-orbiting Isis and Atmospheric Explorer satellites, the
Viking Mars Landers (Hanson et al., 1977) and the Pioneer
Venus Orbiter (Knudsen et al., 1979a, b). A Bennett ion
mass spectrometer achieves ion mass discrimination by selectively accelerating a particular ion mass species with a
radio-frequency (RF) electric field of matching phase speed
(to the speed of the ion mass species) (Bennett, 1950). A
retarding potential analyzer sweeps its retarding potential
grid through a range of potentials, to generate a characteristic curve of the measured ion current as a function of the retarding potential. The shape of the “RPA curve” is typically
fitted to a model to infer the density, bulk velocity, and/or
temperature of the measured ions; under certain conditions
and assumptions, the relative ion composition of the major
ion species can be derived also.
Other early techniques for measuring thermal ion drift
velocities in the ionosphere included the double probe
(Mozer, 1973), which measures the potential difference between two spaced electric field probes to derive the ambient electric field and the corresponding E × B drift velocity; artificial ion cloud, in which the motion of an ion
cloud is tracked using ground triangulation (Wescott et al.,
1969); and cylindrical electrostatic analysis, in which (as
illustrated in Fig. 1 below) a cylindrical electrostatic analyzer is used to measure the differential ion energy-percharge spectrum of the observed ions (Green and Whalen,
1974). The cylindrical analyzer can be combined with ion
time-of-flight analysis to resolve the mass composition of
the ions also (Yau et al., 1981).
Most early particle-counting thermal ion mass and/or energy analyzers used channel electron multipliers, and typically measured ions at one energy or retarding potential at
one incident direction at a time; some measured ions over a
wide and unresolved angular range, and the mass-resolving
analyzers typically sampled one mass species at a time. Notable examples of such analyzers include the retarding ion
mass spectrometer (RIMS) on Dynamics Explorer-1 (Chappell et al., 1981), which used a sector magnet behind a retarding potential analyzer for ion mass separation, and the
suprathermal ion mass spectrometer (SMS) on Akebono
(Whalen et al., 1990), which used a modified Bennett RF
ion mass spectrometer behind a combination of retarding
potential and electrostatic analyzers for ion energy selection
and mass analysis. Both instruments measured ions at one
retarding potential setting in each measurement. In the case
of RIMS, the instrument had a large angle of acceptance,
and sampled two mass steps of a fixed ratio simultaneously
using two separate electron multipliers. In the case of SMS,
which had 3 selectable apertures and corresponding angles
of acceptance, the instrument sampled one mass step in
each measurement and used a 16-pixel micro-channel plate
detector to achieve limited ion energy or angular imaging in
special operation modes (Yau et al., 1998a).
The advent of micro-channel plate detector ushered in
the era of “imaging” plasma analyzer designs, which aim
to resolve the measured plasma in incident angle, energy,
mass, or some combination thereof in a single measurement. Figure 1 compares the ion optics of a cylindrical electrostatic analyzer, which permits ion or electron entry only
at a particular energy-per-charge and one incident azimuth,
with that of the top-hat analyzer pioneered by Carlson et al.
(1983), in which the rotational symmetry about the top-hat
axis allows the simultaneous entry of ions or electrons at a
particular energy-per-charge over the full 360◦ range of incident azimuth. On an attitude-controlled sounding rocket
or orbiting satellite in which the axis of the top-hat analyzer is oriented perpendicular to the local magnetic field,
the top-hat design provides an effective means to image ions
or electrons over the full 180◦ range of pitch-angle. An excellent example of this is the thermal electron capped hemispheric spectrometer (TECHS), which measured the velocity distribution of ∼0.1–10 eV electrons on the SCIFER
sounding rocket (Pollock et al., 1998).
In contrast with the top-hat design, the hemispherical
electrostatic analyzer (HEA) design (Whalen et al., 1994)
allows simultaneous entry of ions or electrons over an ex-
A. W. YAU et al.: IMAGING THERMAL ION MASS AND VELOCITY ANALYZER
205
(a)
(b)
Fig. 2. (a) Sensor cross section view, (b) sensor assembly exploded view.
tended range of energy (instead of a single energy-percharge) and the full 360◦ range of incident azimuth. Figure
6 below depicts the principle of operation of a HEA: basically the analyzer accepts ion or electron entry through an
entrance aperture plane and disperses the incident ions or
electrons by their energy-per-charge while retaining their
incident azimuth, effectively providing a means to image
the 2-dimensional (2D) energy-per-charge and angular dis-
tribution. The basic HEA design was first flown in the
cold plasma analyzer (CPA) on Freja (Whalen et al., 1994)
which measured both electrons and ions in both thermal and
the suprathermal energy ranges (Knudsen et al., 1998).
Since then, the HEA design has been advanced to incorporate a charge coupled device (CCD) detector in the
suprathermal ion or electron imagers (SII/SEI) on several sounding rockets, to provide much denser detector
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Table 1. Sensor bias voltages.
Voltage
VE D
VE S
VE A
VT F
VS A
VF P
VB P
VN C
Definition
Toroidal electrostatic deflector; ±at top and bottom rings
Hemispherical electrostatic analyzer (HEA) outer dome
Entrance aperture bias
Time-of-flight gate electrode; at top and bottom electrodes
Hemispherical electrostatic analyzer (HEA) inner dome
Micro-channel plate (MCP) front surface bias
Micro-channel plate (MCP) back surface bias
Anode
Value/Allowed range (V)
−10 to +10
−10 to +10
0
+10
0 to −353
−4 to −2487
0 to −356
0
40 atomic mass units per charge (AMU/e) on both spinning
and non-spinning spacecraft. An important part of the objective is to resolve the major ion species in the ionosphere
(including H+ , He+ and O+ ) and under certain conditions
+
+
the minor molecular ions (N+
2 , NO and O2 ), and to use the
measured distributions to derive plasma density, drift velocity and temperature parameters.
2.
Fig. 3. Sensor bias voltages defined in Table 1.
pixel density (Knudsen et al., 2003; Burchill et al., 2010).
Both the original Freja CPA instrument and the CCD-based
SII/SEI instruments were purely electrostatic devices and
do not carry any mass-resolution capability. However, the
HEA design has also been advanced to incorporate a timeof-flight gate in the thermal plasma analyzer (TPA) instrument on Nozomi (Yau et al., 1998b); the time-of-flight gate
acted as an embedded ion-mass spectrometer, giving TPA
the capability to resolve the mass identity of the detected
ions and turning it into an ion mass and velocity analyzer
capable of imaging the 2D detailed ion velocity distribution. On the spinning Nozomi spacecraft and on spinning
sounding rocket payloads, successive images of 2D velocity
distributions can be combined to produce a 3D distribution
every half spin.
To measure the 3D velocity phase space distribution of
thermal ions on a non-spinning (3-axis stabilized) platform,
an ion mass and velocity analyzer with electrostatic deflection capability is required to sample ions of arbitrary incident directions relative to the entrance aperture plane of
the analyzer. In the following sections, such a 3D imaging
thermal on mass and velocity analyzer is presented, which
consists of a pair of toroidal electrostatic deflector and a
time-of-flight ion mass spectrometer in addition to a HEA,
and is therefore capable of measuring the mass composition and detailed 3D velocity phase space distributions on a
non-spinning spacecraft. The basic measurement objective
of this instrument is to measure the mass composition and
detailed velocity phase space distributions of thermal- and
suprathermal-energy ions in the energy-per-charge range of
∼1 to 100 eV/e and the mass-per-charge range from 1 to
Principle of Instrument
Like its Nozomi TPA predecessor, the imaging thermal
ion mass and velocity analyzer (thermal ion mass analyzer
or TIM hereafter for brevity) consists of a sensor and an
electronic module. The sensor is housed in a cylindrical
enclosure and is to be mounted to the end of a deployable
boom on a spacecraft, so that it is at a distance from the
spacecraft body and outside of the spacecraft sheath. The
electronic module is mounted inside the spacecraft, and provides all power, digital control, timing, and data interfaces
to the sensor. Figure 2(a) shows a schematic cross section
of the sensor through its axis of rotational symmetry, and
identifies its key functional components, including the deflection rings that form the toroidal the electrostatic deflector, the time-of-flight gates, the inner and the outer electrostatic domes that form the HEA, micro-channel plates, and
the anode printed circuit board (PCB). Figure 2(b) shows
an exploded view of the internal sensor assembly. Figure 3
identifies the bias voltages at the respective surfaces or voltage grids inside the sensor, and Table 1 lists these voltages
and their values or ranges of values. Figure 4 shows a photo
of the sensor and the electronic module of the imaging and
rapid-scanning ion mass spectrometer (IRM) on the Canadian CASSIOPE Enhanced Polar Outflow Probe (e-POP)
satellite (Yau et al., 2006), which is based on the TIM analyzer design.
The ion optics of the sensor is defined principally by its
toroidal electrostatic deflector and HEA. As shown in Figs.
2(a) and 3, the electrostatic deflector consists of a pair of
deflection rings. The top and bottom rings are biased to
selected voltages of opposite polarity (VE D+ and VE D− ), to
deflect incident ions of specific energy-per-charge and elevation combinations into the time-of-flight gate. Figure 5
illustrates the operation of the electrostatic deflector, and
shows the simulated entry of ions of specific energy-percharge and elevation into the sensor in response to the voltage biases at the top and bottom rings (−1.2 V and +1.2
V, respectively). The deflector acts as an energy-elevation
selector in that it accepts lower-energy ions at larger inci-
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207
Fig. 4. Sensor (foreground) and electronics module (background) of the imaging and rapid-scanning ion mass spectrometer (IRM) on the CASSIOPE/e-POP satellite.
Fig. 5. Simulated trajectories of incident ions at 4-60 eV/e and 0◦ -60◦ elevation, in response to the voltage biases at the top and bottom deflection rings
of the sensor.
dent elevations and higher-energy ions at smaller elevation.
Note that when both the top and bottom deflection plates are
set to ground, the sensor samples only ions at 0◦ elevation,
and measures the 2D velocity distribution in the entrance
aperture plane.
The hemispheric electrostatic analyzer (HEA) consists
of an inner and an outer hemisphere (“dome”). The inner dome is biased negative relative to the outer dome
(VS A < VE S ), and the resulting central electric field disperses the incident ions by their energy-per-charge and incident azimuth onto the micro-channel plate detector: the
landing radius and azimuth of each detected ion on the MCP
surface maps to the ion energy and azimuth of arrival. Figures 6(a) and (b) illustrate the dispersion of ions through
the HEA by their energy-per-charge and azimuth. In other
words, the HEA focuses incident ions of a given azimuth
and energy per charge onto a single point on its hemispherical plane, essentially irrespective of the ion elevation angles
and entrance positions. The radial distance of this point increases with ion energy-per-charge. The maximum ion energy sampled depends primarily on the voltage difference
(VS A − VE S ). Figure 6(a) shows the side view of simulated
trajectories of incident ions of 4–60 eV/e, when VS A was set
to −200 V relative to VE S . The highest energy ions arrive
at the outermost portion of the MCP. Figure 6(b) shows the
top view of simulated trajectories of incident ions of lower
energies (1–15 eV/e), when VS A was set to a correspondingly lower value. The highest energy ions again arrive at
the outermost portion of the MCP, and all of the ions arrive
at the opposite azimuth regardless of their energy.
The time-of-flight (TOF) gate consists of a pair of fastswitching electrodes, which are controlled by the TOF gate
driver, and it opens and closes repeatedly in a measurement
to allow ion entry. Figure 7 shows an exploded view of the
internal assembly of the TOF electrodes, which are made of
a set of 3 concentric rings of foil, and Fig. 8 illustrates the
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(a)
(b)
Fig. 6. (a) Side-view of simulated trajectories of incident ions at 4–60 eV/e dispersed by the HEA onto the MCP detector, with the higher-energy ions
arriving at the outermost portion of the MCP. (b) Top-view of simulated trajectories of incident ions at 1–15 eV/e being dispersed by the HEA onto
the MCP detector, with the ions arriving at the opposite azimuth of the MCP.
operation of the TOF gate and its open-and-close sequences
in a measurement. Figure 8 depicts the multiple TOF cycles
in a measurement period (top panel) and the time-of-flight
gate open and close sequence in each TOF cycle (bottom
panel). Table 2 lists the ranges of timing parameters in the
TOF gate. Each TOF cycle consists of 1024 time-of-flight
bins, each of 40 ns duration (N B I N = 1024; TB I N = 40
ns). In other words, a TOF cycle is 40.96 µs in duration. A
measurement consists of a number of TOF cycles, N T O F ,
which can have a value of 1 to 65535; corresponding to a
measurement period of 0.4 to 25.6 ms. The default value of
N T O F is 240 and corresponds to a measurement period of
9.83 ms (i.e. 100 measurements per second). In each TOF
cycle, the TOF gate can remain open for up to 255 TOF bins
(N O P N = 0 − 255); however, in practice, N T O F is typically
between 10 to 50, which corresponds to a duty cycle of 1.0–
1.5% and a gate-open period of 0.4 to 2 µs.
The detector consists of a pair of micro-channel plates
(MCP) and a discrete anode. An incident ion that is passing
through the HEA produces a charge as it arrives at the front
surface of the MCP. The charge is amplified through successive collisions with the channel walls inside the MCP into
a “charge cloud”, which is proximity focused at the back of
the MCP and then slightly defocused before being collected
onto the anode. The front side of the anode contains 64 discrete pixels, which have radial widths from 0.33 to 1.6 mm
and arc lengths from 2.8 mm to 1.0 mm, and arranged in
8 rows (rings) and 8 columns (spokes). The defocusing is
to increase the spatial spread of the cloud, from tens of microns to hundreds of microns, to distribute the charge over
a larger fraction of a pixel’s surface area. On the backside
of the anode, each pixel is connected to a pair of charge
amplifiers, which detect the charge buildup (“particle hit”)
at the pixel above a threshold value (2.9 × 105 electrons).
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Fig. 8. Top: TOF gate open and close sequences in a measurement period
consisting of multiple TOF cycles; bottom: Time-of-flight gate (TOF)
open-and-close sequence in a TOF cycle.
Fig. 7. Time-of-flight gate electrode assembly.
In order to direct roughly half of the charge hitting a pixel
to each of the two charged amplifiers, each pixel is subdivided into two interlaced, separated and electrically isolated
halves, and each half is connected to one of the two amplifiers, so that statistically each amplifier is likely to detect
half of the charge cloud. A consequence of the interlaced
pixel subdivision is that whenever the split of a charge cloud
is very unequal between the two amplifiers the charge may
be detected above the threshold level at only one of the two
amplifiers.
On the adjacent pixel encoder board, the circuitry of preamplifiers identifies the row and column pixel address of
each detected ion and its arrival time relative to the TOF
gate opening. A field programmable gate array (FPGA) encodes the 6-bit pixel address and a 10-bit time-of-flight bin
number into a 2-byte data word and subsequently writes it to
a first-in-first-out (FIFO) data buffer. In addition, the FPGA
tracks the number of such data words, which corresponds to
the number of “hits” with definitive pixel row and column,
as well as the number of “detects”, i.e. events of ion detection without definitive pixel address information such as
those detected by only one preamplifier. In detailed laboratory testing, the “hit-to-detect” ratio was found to be in the
range of 0.25 to 0.4, depending on the spatial spread of a
charge cloud and its precise location on the pixel. At the
end of each measurement period, the FIFO data words are
sent to the digital signal processor in the electronics module,
where the sensor data are processed, as will be discussed
later in this section. For the given sensor voltage setting
in a measurement, one can unambiguously determine the
energy-per-charge, mass-per-charge, and incident azimuth
and elevation of each detected ion.
The optimum mounting orientation of the sensor on a
non-spinning spacecraft is naturally dependent on the at-
titude of the spacecraft. From the foregoing, it can be seen
that on a nadir- or ram-pointing spacecraft, the entrance
aperture plane of the sensor should ideally be aligned in
the spacecraft ram-and-nadir plane (X-Z plane where the
Z-axis is in the nadir direction and the X-axis is in the ram
azimuth in the horizontal plane). When the spacecraft is
nadir-pointing, the sensor will sample ions at all elevation
angles at both the ram and anti-ram (wake) azimuth including the upward and downward vertical directions. Likewise,
when the spacecraft is ram-pointing, the sensor will sample
ions in the ram, anti-ram, and vertical directions. It should
be noted here that the terms elevation and azimuth refer to
the angle from the local horizontal (positive upward) and
the angle from the east (positive anti-clockwise) in the local horizontal plane. On a spinning spacecraft, the entrance
aperture plane of the sensor should ideally be aligned with
the spacecraft spin axis, so that the sensor will sample all directions (4π of solid angles) every half a spacecraft spin. It
is important to keep in mind here that in the case where the
sensor entrance aperture plane is aligned with the local vertical, a one-to-one mapping exists between a sensor azimuth
(an azimuth in the sensor entrance aperture plane) and the
corresponding elevation and azimuth of its look direction
with respect to the local vertical and horizontal directions.
Figure 9 shows a schematic block diagram for the sensor, including the architecture of the time-of-flight (TOF)
gate driver, anode, and pixel encoder card, and the series of
high and low voltage grids. Figure 10 shows a schematic
block diagram of the electronics module, which consists of
a digital signal processor, a low voltage power supply, and a
high voltage power supply. The digital signal processor acts
as the command and data interface with the spacecraft, and
controls the operations of both the high voltage power supply and the sensor. The low voltage power supply converts
the spacecraft bus power (+28 Vdc) into regulated and isolated +3.3 Vdc, +5 Vdc and ±12 Vdc outputs. The high
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Table 2. Time of flight gate timing parameters in a measurement.
Parameter
TB I N
NB I N
NO P N
NT O F
Definition
Duration of 1 TOF bin
# of TOF bins in a TOF cycle
# of gate-open TOF bins in a TOF cycle
# of TOF cycles in a measurement
Value/Range
40 ns
1024
0–255
1–65535
Typical value
10–15
240
Fig. 9. Sensor block diagram.
voltage power supply provides the sensor voltages and their
ground reference. Figure 11 shows the key dimensions of
the sensor, which measures 94 mm in diameter and 164 mm
in height and weighs 1.30 kg; the electronic module measures 203 × 185 × 65 mm3 and weighs 1.83 kg, and consumes 6.2 W of peak power and 4.7 W of standby power at
a spacecraft bus voltage of 28 V.
The instrument is capable of operating in a number
of pre-programmed “measurement modes” in response to
ground command. The control software for these measurement modes is stored in memory in the digital signal processor. In each measurement mode, the sensor can step repeatedly through a sequence of up to 4 sets of sensor settings (bias voltages and TOF gate timing parameters), and
repeat each set of settings up to 255 times (measurements).
Thus a measurement mode consists of a sequence of up to
1020 measurements. As can be seen in Table 2, a measurement period is typically 10 ms but can be as long as
2.68 s. In each measurement period, the sensor settings
remain fixed and correspond to a fixed range of sampled
ion mass-per-charge and a fixed region (subset) of the ion
energy-per-charge and incident elevation phase space, and
the sensor can transmit up to 65,536 bytes of data to the
digital signal processor for processing. At the end of each
sequence of measurements, the pixel data in each measurement period are packetized without any onboard data reduction and transmitted to ground whenever the telemetry
bandwidth permits; at other times, the data are processed
onboard: the pixel data are summed over all measurement
periods of each sensor setting to create ion count histograms
or moments at selected detector anode pixels and time-offlight bins to reduce the telemetry volume. Although the
instrument is in principle capable of generating pixel data
at a maximum rate of about 13 megabytes per second, the
actual maximum data throughput rate is much lower, and is
about 0.5 megabytes per second or 4 megabits per second.
3.
Sensor Response
A series of calibration measurements were made using
the charged particle calibration facility at the University of
Calgary to characterize the angular, energy, mass and sensitivity responses of the sensor to low-energy ions. Two different low-energy (1–200 eV) ion sources were used: the
first one has a smaller beam area (2 cm × 2 cm) and a
beam width of ±2◦ and was used previously to calibrate
the Akebono suprathermal mass spectrometer (Whalen et
al., 1990), and the second one has a larger beam area (10
cm × 10 cm) and a higher output beam current (up to 1
A. W. YAU et al.: IMAGING THERMAL ION MASS AND VELOCITY ANALYZER
211
Fig. 10. Electronics module block diagram.
nA). In both cases, a faraday cup was used for absolute
flux calibration and for beam energy and width characterization. Both the sensor and the faraday cup were mounted
on a manipulator table, to facilitate sensor rotation in 2 axes
and translational movement in 2 directions with respect to
the ion source. In all measurements the energy of the ion
beams had a typical FWHM of ≈30–50%. Molecular nitrogen was used in most of the calibration runs because of
its chemically inert properties; Argon was used in the remaining runs. The majority of the calibration runs were
performed using ions between 10 and 50 eV, which in the
case of N+
2 corresponds to a velocity of 8.3 to 18.6 km/s,
to mimic the velocities of both cold and accelerated ionospheric ions seen by the sensor in orbit: since the spacecraft
ram velocity in low-Earth-orbit is ∼8 km/s, a co-rotating
ion will have a relative velocity of ∼8 km/s in the sensor
frame of reference.
Ideally, it is desirable to characterize the detailed sensor response by using a highly collimated and energyselectable, mono-energetic ion source of a selectable mass
species and systematically varying its orientation relative to
the sensor (or vice versa) and the selected ion energy, to
determine the geometric factor, G(E, ), at each detector
pixel as a function of incident ion energy E and angle for a given set of sensor (voltage and timing parameter) settings; = (θ, φ) and θ and φ are elevation and azimuth,
respectively. However, because of the very large number
of possible sensor settings (combination of voltage parameters in Table 1 and time-of-flight gate timing parameters in
Table 2), it is not possible to calibrate each and every sensor setting in this manner. Moreover, as noted above, the
energy spread (FWHM of 30–50%) of the ion sources was
large compared with the intrinsic sensor energy response
(E/E of ≈10–15%) particularly at low energy. Therefore, calibration measurements were made only at a number
of selected sensor settings and incident ion beam energies,
angles and fluxes to determine the averaged sensor response
at these settings over the broad energy range of the source
ion beam.
To determine the detailed sensor energy response function, i.e. G(E, ) at each pixel as a function of E and and the corresponding energy resolution (E/E) and angular range (angle of acceptance; ), a particle-tracing
code was used to compute the electric field distribution inside the sensor and in the vicinity of its entrance aperture
as a function of the selected sensor settings, and the corresponding ion trajectory as a function of incident ion energyper-charge, mass-per-charge, elevation and azimuth. The
simulated trajectories for the selected ion beam energies,
angles and fluxes were then used to calculate the predicted
response of the sensor at the selected sensor settings, for
comparison with the calibration data. Based on the generally good agreement (a few percent) between the simulated
and measured responses, the simulation code was then used
for computing the sensor response function at all sensor settings, and the computed response will be used to convert inflight ion count data to ion fluxes and velocity phase space
densities for detailed analysis.
Figure 12 shows the measured dependence of the incident elevation of sampled ions on the toroidal electrostatic
deflector voltage for nitrogen ions (N+
2 ) at 15 eV. In this calibration run, the sensor was oriented so that the sensor axis
was orthogonal to the ion beam and one of its anode pixel
columns was aligned with the beam, and the toroidal deflec-
212
A. W. YAU et al.: IMAGING THERMAL ION MASS AND VELOCITY ANALYZER
Fig. 11. Sensor dimensions of imaging thermal ion mass and velocity analyzer.
tion voltage (VE D ) was set to 0 V; VS A was fixed to −69.5 V,
to ensure that the incident ions would arrive at the third outermost pixel (“pixel 6”). The elevation angle was then varied at 5◦ steps. At each step, the number of ions detected in
the pixel (“hits”) was measured as a function of the varying
deflection voltage, and the measured profile fitted to a Gaussian to determine the corresponding “optimum” deflection
voltage. Figure 12 shows a linear relationship between the
elevation angle θ and deflection voltage amplitude VE D except at large elevation angles: i.e. θ = αVE D + β where
θ is in deg, VE D is in volt, α = 11.1◦ /V and β = 0.9◦ .
Such a linear relationship was found to hold in other calibration runs at other ion energies or using argon ions (Ar+ )
as well; in each case, the numerical value of α was found
to be in good accord with the simulation and increase with
decreasing incident ion energy, and the value of β was ∼1◦
and is attributed to the relative misalignment between the
sensor and the ion source. Thus the measurements confirm
the ability of the sensor to sample low-energy (1–15 eV)
ions up to 60◦ off the sensor entrance aperture plane on a
non-spinning platform.
Figure 13 shows selected measured time-of-flight spectra
of 50 eV N+
2 ions. The top and bottom panels show spectra
at 1 and 0.1% TOF duty cycle, respectively; in each case,
the right panel shows the data on an expanded TOF scale
to reveal the detailed profiles of measured TOF peaks. In
both cases, VS A was set to −300 V, resulting in the ions
being detected at the second outermost pixel (“pixel 7”) in
the anode pixel column. In both cases, two TOF peaks were
observed; in contrast, only one TOF peak was observed in
the runs using argon ions. The larger peak starting near
TOF bin 54 is attributed to N+
2 ions while the smaller peak
starting near TOF bin 40 is attributed to N+ ions, based on
the analysis below.
The TOF bin value of a measured ion is determined by
Fig. 12. Measured dependence of the incident elevation of sampled ions
on the torodial electrostatic deflector voltage for ions at 15 eV.
the time delay between the start of the time-of-flight gateopen period in a TOF cycle and the time of arrival at the
6-bit pixel address of the detected ion at the FPGA. This
time delay τ is comprised of 4 components: the time of ion
transit from the outboard edge of the TOF gate to the outer
“dome”, τ1 ; the corresponding time from the outer dome
to the front of the MCP detector, τ2 ; the transit time of the
electron charge cloud through the MCP to the anode pixel,
τ3 ; and the time delay of the pre-amplifier firing, τ4 . For
a given ion energy, τ1 depends on VE S and is essentially
proportional to the square root of the ion mass in the case
of VE S = 0, and τ2 depends on both the ion mass and the
magnitude of VS A ; τ3 is on the order of 1 ns and therefore
negligible compared with τ1 and τ2 . The quantity τ4 is dominated by the triggering time of the preamplifiers, which is
dependent on the rate of charge injection and the pulse am-
A. W. YAU et al.: IMAGING THERMAL ION MASS AND VELOCITY ANALYZER
213
Fig. 13. Measured time-of-flight (TOF) spectra of 50 eV N+
2 ions: top: 1% TOF duty cycle (TOF gate open period = 10 TOF bins = 0.4 µs); bottom:
0.1% TOF duty cycle (TOF gate open period = 1 TOF bin = 0.04 µs); left: full TOF scale; right: expanded TOF scale to show the detailed profiles
of observed TOF peaks.
plitude of the injected charge, and was found to be about
220 ns under typical sensor operating conditions. Including the rise time of ∼25 ns and the fall time of ∼80 ns, τ4
is about 320 ns or 8 TOF bins. Thus, for a given sensor
voltage setting (VS A and VE S ) and ion energy, the starting
TOF bin number of a measured ion is approximately given
by (C(m/q)1/2 + 8) where C is a proportionality constant,
and m/q is ion mass-per-charge. In the case of 50 eV ions,
VS A of −300 V and VE S of 0 V in Fig. 13, C is approximately 8.86 (AMU/e)−1/2 from simulation. Thus, the TOF
+
peaks for mass 28 and 14 (N+
2 and N ) ions are predicted to
start near TOF bin 55 and 41, respectively. In other words,
the observed TOF peaks in Fig. 13 are consistent with the
+
measured ions being N+
2 and N ions, respectively.
In Fig. 13, the widths of the TOF peaks in the top panels
are discernibly different from those in the bottom panels.
This difference is attributed to the different TOF gate period
in each case, as follows. As shown in Fig. 13, the TOF gate
electrode has a width of 0.7 mm. Therefore, it would take a
+
50-eV N+
2 ion about 0.038 µs and a corresponding N ion
about 0.027 µs to traverse the electrode. In comparison, the
TOF gate remained open for 1% of the TOF cycle in the top
panels, i.e. 0.4 µs or 10 TOF bins of 40 ns each. Therefore,
all of the ions arriving at the TOF gate within the first 9 of
the 10 TOF bins after the gate opening will have sufficient
time to traverse the electrode before the gate closes: this
explains the relatively flat peaks spanning several TOF bins
in the top panels. In contrast, in the bottom panel, the TOF
gate remained open for only 0.1% of the TOF cycle or 0.04
µs, which is comparable to the transit time of the N+
2 ions
through the TOF gate electrode. Therefore, a substantial
fraction of ions arriving at the TOF gate during the gate
open period failed to exit the TOF gate before the end of
the gate-open period: this explains the much narrower TOF
peaks in this case. Since cold ionospheric ions will appear
as ions with a relative velocity of ∼8 km/s in the sensor
frame of reference on a low-Earth-orbit satellite, it can be
seen from the results in this figure that they will have a
transit time on the order of about 0.1 µs through the TOF
gate. Thus, the TOF gate must remain open for at least 4 or
5 TOF bin periods to measure such ions.
The results in Figs. 12 and 13 and similar results from
other calibration runs combine to demonstrate that the fact
that as discussed above, the ion optics of the sensor is defined principally by the voltage settings at its toroidal electrostatic deflector and HEA (VE D , VE S and VE A ). The body
of calibration results also confirms that as can be seen in
Fig. 6, the overall ion optics is also affected by the voltage
settings at the front and back of the MCP detector (VF P ,
VB P ), albeit to a lesser extent, while the mass response is
controlled by the voltage settings and timing parameters of
the time-of-flight gate (in particular VT F , N O P N ).
214
4.
A. W. YAU et al.: IMAGING THERMAL ION MASS AND VELOCITY ANALYZER
Summary and Discussion
The ion mass composition and detailed velocity phase
space distributions of thermal (low-energy) plasma often
play an important role in the structure and the dynamics
of a planetary ionosphere or magnetosphere. The aim of an
imaging thermal ion mass and velocity analyzer is to apply
imaging techniques to measure in-situ both the mass composition distribution of a thermal plasma population and the
mass-resolved velocity phase space distribution of each of
its ion species, and to use the measured distributions to derive the bulk plasma parameters and to detect the possible
presence of non-thermal distributions in a planetary ionosphere or magnetosphere.
A hemispherical electrostatic analyzer (HEA) simultaneously samples incident ions or electrons over an extended
energy range and the full 360◦ range of incident azimuth in
its entrance aperture plane, and disperses the ions or electrons by their energy-per-charge while retaining their incident azimuth, effectively providing a means to image the 2D
energy-per-charge and azimuth distribution. Thus a HEA
with an embedded ion-mass spectrometer is capable of resolving the mass identity of the measured ions and imaging the 2D detailed velocity phase space distributions of the
different ion species, and effectively becomes an imaging
ion mass and velocity analyzer; on a non-spinning (3-axis
stabilized) platform, an analyzer that has additional electrostatic deflection capability is capable of measuring the 3dimensional velocity distributions also. This is in contrast
to a top-hat analyzer, which also samples particles over the
full range of azimuth and retains their individual azimuth
but is restricted to sampling ions or electrons at only a single energy at a time.
In the case of the imaging thermal ion mass and velocity
analyzer presented above, a toroidal electrostatic deflector
is used to sample ions at different elevations (angles to the
sensor entrance aperture plane) and a time-of-flight gate
is used to measure the individual ion time-of-flight. The
instrument is designed to measure thermal-energy ions in
the energy-per-charge range of ∼1 to 100 eV/e and the
mass-per-charge range of 1 to >40 atomic mass units per
charge (AMU/e) at up to ∼10% energy resolution (E/E)
and ∼5◦ angular resolution, and to resolve all major ion
species in the ionosphere including H+ , He+ , O+ as well as
+
+
adjacent molecular ion species such as N+
2 , NO and O2
under favorable conditions.
Compared with the top-hat and other plasma analyzers, imaging analyzers are capable of measuring an ion or
electron energy distribution at a much faster rate (up to
∼100 Hz), making them ideally suited for studying temporal structures or plasma processes down to time scales of
tens of ms or small scale plasma structures down to spatial
scales of tens of meters on a sounding rocket or hundreds of
meters on an orbiting satellite.
As noted in the Introduction section, both mass-resolving
and non-mass-resolving imaging thermal plasma analyzers
currently exist, and they often offer complementary advantages. For example, the analyzer above provides definitive
ion mass species identification and velocity determination,
which are crucial for certain scientific investigations such
as the mass dependence of various ion acceleration mech-
anisms, but its detector has relatively limited pixel resolution (density). In comparison, the suprathermal ion analyzer flown on the GEODESIC and JOULE sounding rockets (Knudsen et al., 2003), which is also based on the HEA
design, uses a CCD detector to provide much denser detector pixel density, thereby providing 2D ion energy distributions of much higher energy and angular resolution, while
requiring assumptions on ion composition to be made in
detailed velocity analysis due to the lack of any ion mass
resolution capability.
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